The present invention relates generally to a digital rendering system and, more particularly, to an apparatus and improved method for measuring color registration and determining registration error in a multicolor marking platform.
A digital rendering system renders a digital image, consisting of electronic pixel data, to a human readable version of the image. Digital rendering systems typically include: 1) an input section, sometimes referred to as an image input terminal (xe2x80x9cIITxe2x80x9d) or a digital front end (xe2x80x9cDFExe2x80x9d), 2) a controller, sometimes referred to as an electronic subsystem (xe2x80x9cESSxe2x80x9d) or an image processing system (xe2x80x9cIPSxe2x80x9d), and 3) an output section, sometimes referred to as an image output terminal (xe2x80x9cIOTxe2x80x9d).
The input section either generates or translates one form of image data to digital image data that can be provided to the controller. The input section can be a scanner, individual computer, distributed computer network, electronic storage device, or any device capable of generating or storing the digital image. The controller processes the digital image data to create machine readable image data that is compatible with the output section. The controller also controls operations within the output section. The output section receives machine readable image data from the controller and produces a human readable version of the digital image. The output section can be a display device (e.g., cathode ray tube (xe2x80x9cCRTxe2x80x9d) monitor), marking platform (e.g., copier or printer), electronic storage device, or any device capable of producing a human readable image. For marking platforms, the output section is sometimes referred to as a print or marking engine. Again in reference to marking platforms, the human readable image is created by depositing marking material on a print page. The print page is often a single sheet of white paper, however, numerous other materials are available. Two technologies commonly used in marking platforms are ink marking and toner marking. Ink-jet printers and offset printing presses are common examples of platforms that implement ink marking technology. Marking platforms that use toner marking include electrophotographic printers, copiers, and multifunction peripherals. Toner marking is also known as electrophotographic marking.
Generally, electrophotographic marking employs a charge-retentive, photosensitive surface, known as a photoreceptor. The photoreceptor is a photoconductive region on the surface of a rotating drum or moving belt. The photoconductive region may only have one imaging region or there may be multiple imaging regions, particularly for the belt. The electrophotographic process begins by applying a uniform charge to the photoreceptor. In an imaging and exposing step, a light image representation of a desired output is focused on the photoreceptor and discharges specific areas of the surface to create a latent image. In a developing step, toner particles are applied to the latent image, forming a toner or developed image on the photoreceptor. This toner image is then transferred to a print page. The toner particles are heated to permanently affix the toner image to the print page. Finally, the photoreceptor passes through a cleaning step to prepare it for another electrophotographic cycle.
The electrophotographic marking process outlined above can be used to produce color as well as black and white (monochrome) images. Generally, color images are produced by repeating the electrophotographic process to print two or more different image layers or color image separations in superimposed registration on a single print page. Commonly, full color copying or printing is provided by subtractive combinations of cyan, magenta, and yellow toner. Such color mixing to produce a variety of colors is called process color separation. To produce black, a combination of equal amounts of cyan, magenta and yellow toner layers are mixed, or a fourth black color toner layer may be used as a substitute. To extend the color gamut of the process color output, as in high fidelity copiers or printers, additional colors of toner (e.g., red, green, blue, or a customer-selectable color) may be used in combination with the three or four color separations. Alternatively, where the additional colored toner is not mixed with the process colors, referred to as spot color separation, the additional toner image layer is used to set off certain portions of text or graphics in the composite full color image. Color images can also be produced with two colors, such as black and red, by a highlight color copier or printer. Highlight color copying or printing can use process color separation to produce images similar to halftone or gray-scaled images or spot color separation to produce images with the two colors unmixed.
A multicolor electrophotographic process may be accomplished by either a multiple pass or single pass marking engine. Marking platforms with a multiple pass marking engine require less hardware and are generally easier to implement than systems with a single pass marking engine. However, systems with a single pass marking engine provide much greater throughput.
A multiple pass marking engine uses multiple cycles of the electrophotographic process described above, one cycle for each color separation. During each cycle, a toner image layer for one color separation is formed on the photoreceptor and transferred to an intermediate substrate or to the print page. In the multiple pass architecture described, the composite color image is accumulated on the intermediate substrate or the print page in successive electrophotographic cycles. In the case where the image is accumulated on an intermediate substrate, the composite toner image is transferred to the print page and then fused. In the case where the composite toner image is accumulated on the print page, the composite toner image is fused after the last color separation is transferred. In an alternate multiple pass architecture, the composite color image is accumulated on the photoreceptor in multiple cycles and, after the last toner image layer is developed on the photoreceptor, the composite toner image is transferred and fused on the print page.
A single pass marking engine employs multiple charging, imaging and exposing, and developing devices, one set for each color separation. In the single pass architecture, each color separation device set sequentially applies a toner image layer to the photoreceptor. Within each color separation device set, the electrophotographic steps of charging, imaging and exposing, and developing occur as described above. The composite color image is accumulated on the photoreceptor in a single electrophotographic cycle in the single pass marking engine. The composite color image on the photoreceptor is then transferred and fused to the print page.
Another type of single pass marking engine, often referred to as a tandem architecture, employs multiple photoreceptors in addition to the components of the previously described single pass marking engine. In the tandem architecture, each color separation has a set of charging, imaging and exposing, developing, and photoreceptor devices. Additionally, an intermediate transfer belt in the tandem architecture accumulates the individual toner images in a composite image in a manner not unlike the single photoreceptor in the previously described single pass architecture. Each photoreceptor sequentially transfers a toner image layer to the image area on the intermediate transfer belt. The composite color image is accumulated on the intermediate transfer belt in a single electrophotographic cycle and then transferred and fused to the print page.
While the color imaging techniques described above are discussed primarily in reference to electrophotographic marking, they are also applicable to ink marking and any other type of marking platform that creates a composite multicolor image by combining multiple color separation layers. In multicolor marking platforms that form and transfer individual color separation images in superimposed registration with one another, proper or precise registration of individual color images is usually an important and difficult problem. In order to deliver good quality composite color images, strict specifications are imposed on the accuracy with which a multicolor marking engine superimposes the various color separations. Registration errors, for example, can arise from motion errors of the image receiving members or from alignment errors of individual color separation development members. Techniques for improving registration in marking platforms often involve placement of registration indicia marks on a surface, detection of such marks, assessment of registration conditions, determining the degree of registration error when a misregistration condition exists, and controlling marking platform members to correct for registration errors. Detection systems, both mechanized and human are used to measure color registration and determine registration error in both the process direction and lateral (cross-process) direction.
One common way of improving color registration, as described in U.S. Pat. Nos. 5,287,162 and 5,748,221 and incorporated herein by reference, is to use bi-level detectors to measure the diffuse reflectance of a sequence of color registration marks, such as chevron marks. Process and lateral (cross-process) registration errors are determined from timing functions which relate the velocity of the image receiving members to the time centroids of the chevron marks pass by the detectors. This technique requires precise construction and alignment of detectors, precise measurement of diffuse reflectance, precise measurement or precise control of the velocity of image receiving members, and significant processing overhead to perform color registration measurements and determine registration error.
Another common way of improving color registration, as described in U.S. Pat. Nos. 5,574,527, 5,839,016, and 5,909,235 and incorporated herein by reference, is to use an optical sensor to measure the diffuse reflectance of either a sequence of color registration marks or a set of multicolor test patches. When color registration is detected using a sequence of color registration marks, process and lateral (cross-process) registration errors are determined based on timing functions much like the chevron marking technology described above. This technique requires precise measurement of diffuse reflectance, precise measurement or precise control of the velocity of image receiving members, and significant processing overhead to perform color registration measurements and determine registration error.
When color registration is detected with the optical sensor using a set of multicolor test patches, at least four, preferably ten, test patches are required for each non-black color to be registered. Each multicolor test patch is comprised of a set of color registration marks on a set of black registration marks in a predetermined registration condition. For each predetermined registration condition, an expected reflectance value and a registration error are known. At least two, preferably five, test patches are grouped to measure process registration. Likewise, at least two, preferably five, test patches are grouped to measure lateral (cross-process) registration. The actual reflectance of each patch is measured and compared to an expected value corresponding to the predetermined registration condition of the patch. Actual registration error in the process direction is estimated by comparing the measured and expected values for the process group of test patches and interpolating between or extrapolating from the known process registration error for the predetermined registration conditions. Actual registration error in the lateral (cross-process) direction is estimated in the same manner using the lateral (cross-process) group of test patches. This technique requires precise measurement of reflectance and significant processing overhead to perform color registration measurements and determine registration error.
Specular and diffuse reflectance are parameters that have been used for some time to control various processes in marking platforms, including toner concentration, developability, discrimination between opaque paper and transparencies, and gloss. However, the reflectance, and the corresponding density, of a test pattern has not will be been used as a parameter for controlling the registration of colors in a multicolor marking platform. Techniques for measurement of color registration and subsequent determination of registration error can be improved over prior techniques by employing an apparatus and method which employs the reflectance or density of multicolor test patterns as a control parameter. A need also exists for streamlining the color registration process by reducing the spatial precision necessary in measuring color registration in a multicolor marking platform and by reducing the processing overhead necessary for determining registration error.
The present invention provides a multicolor marking platform with a color registration measurement system that uses either the reflectance or the density of a test pattern to determine the registration of one or more non-black colors in reference to a black color.
The present invention also provides a method of determining the degree of color registration error of the non-black colors in reference to the black color from the density of the test pattern.
An advantage of the present invention is that it is applicable to any marking platform that develops multicolor images in color separation layers that are in superimposed registration with one another, including but not limited to electrophotographic and ink marking platforms.
Another advantage of the present invention is that it requires less measurement precision than prior color registration methods.
Another advantage of the present invention is that its color registration measurements can be performed on images formed on a photoreceptor, on images assembled on an intermediate substrate, on images transferred to a target substrate, and on images fused to a target substrate.
Another advantage of the present invention is that it requires less processing overhead than prior color registration methods while also providing a more precise method of estimating color registration error.